Request keyframes more frequently on stream start/decoding error.

In this CL:
 - Added FrameObject::is_keyframe() convinience function.
 - Moved logic to request keyframes on decoding error from VideoReceived to
   VideoReceiveStream.
 - Added keyframe_required as a parameter to FrameBuffer::NextFrame.

BUG=webrtc:8074

Review-Url: https://codereview.webrtc.org/2993793002
Cr-Original-Commit-Position: refs/heads/master@{#19280}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 26b48043581735eed6e36b95fae6f5b1bcf8cfb5
7 files changed